We explore links between performance under electrical stress and the way in
which charge is localised in two contrasted systems. Both systems are arnorphous. In one case inhomogeneity which plays a role; for the other, it is topology. For silicon dioxide, the gate dielectric of microelectronics, we show that the charge state of the oxidising species (mobile atomic or molecular oxygen) is critical for ultra-thin oxides. The surprises are firstly that the differences between crystalline and amorphous oxides are substantial, and secondly the way that the important charge states should vary systematically through the oxide thickness. We identify ways in which the oxide performance might be improved. For an amorphous polymer.. we establish that the mesostructure (spaghetti structure) of the polymer can have significant effects on its performance, both for insulating and conducting polymers. In the case of conducting polymers, the surprise is that modelling shows that, even for “trap-free” cases, electrons can be trapped for times long compared lo average transit times because the local configuration other carriers obstructs the direct routes lo the anode. In neither case is a simple continuum space charge description valid. We also present: results from self-consistent molecular dynamics which show that electronic excitation can cause local rebonding in ways which appear to resemble the initiation electrical breakdown.

The aim of this study was to evaluate the clinical performance (retention rate) of fiber-reinforced composite fixed partial dentures (FPDs). Polyethylene fiber (Ribbond®) was used combined with restorative composite during FPDs fabrication. FPDs were placed in thirteen patients in a private clinic. Nineteen FPDs were evaluated. The prosthetic space was filled with only one pontic using extracted teeth (2 cases), acrylic resin teeth (11 cases), or with composite resin (6 cases), combined with Polyethylene fiber. The clinical criterion used was based on retention rate of FPDs. If FPDs were in function in the mouth at the time of examination without previous repair they were classified as Complete Survival (CS) restorations. A classification of Survival with Rebonding (SR) was assigned in the event of an adhesive failure, but after rebonding the FPD still remained under evaluation. Treatment was classified as a Failure (F) if the FPD restoration was lost. The time of evaluation was 41.15 months (±15.13). The FPDs evaluated were retained (CS=94.75%), and no failure was found except for in one situation which required rebonding (SR=5.25%). According to the survival estimation method of Kaplan-Meyer the mean survival time was 42.3 months. At the time of evaluation investigated...

Objective: To compare the conventional etching and primer method (CEP) and the self-etching primer method (SEP) in rebonding brackets. Methods: Forty human maxillary second premolars extracted for orthodontic purpose were randomly divided into 4 equal groups. Group 1 and Group 2 were bonded using the CEP method; Group 3 and Group 4 using the SEP method. All the brackets were debonded and 40 new brackets were rebonded with four different protocols after surface cleaning: Group 1: CEP + adhesive; Group 2: CEP without etch step + adhesive; Group 3: SEP + adhesive; Group 4: non-acidic primer + adhesive. Then, the shear bond strength (SBS) of each group was tested and the measurements of adhesive remnant index scores (ARI) and SEM examination were performed. Results: The mean SBSs for Group 1, 2, 3 and 4 were 14.18, 6.57, 11.90, 5.91 MPa, respectively. Statistical differences of the SBS existed between Group 1 and 2 (P < 0.05) and between Group 3 and 4 (P < 0.05). No difference was found between Group 1 and 3, or Group 2 and 4. Conclusion: Omission of the acid-etching step in rebonding orthodontic brackets may be adequate for the clinical requirement. No differences in SBS and ARI of the rebonded brackets were showed between CEP and SEP methods.

The aim of this study was to evaluate the clinical performance (retention rate) of fiber-reinforced composite fixed partial dentures (FPDs). Polyethylene fiber (Ribbond®) was used combined with restorative composite during FPDs fabrication. FPDs were placed in thirteen patients in a private clinic. Nineteen FPDS were evaluated. The prosthetic space was filled with only one pontic using extracted teeth (2 cases), acrylic resin teeth (11 cases), or with composite resin (6 cases), combined with Polyethylene fiber. The clinical criterion used was based on retention rate of FPDs. If FPDs were in function in the mouth at the time of examination without previous repair they were classified as Complete Survival (CS) restorations. A classification of Survival with Rebonding (SR) was assigned in the event of an adhesive failure, but after rebonding the FPD still remained under evaluation. Treatment was classified as a Failure (F) if the FPD restoration was lost. The time of evaluation was 41.15 months (±15.13). The FPDs evaluated were retained (CS=94.75%), and no failure was found except for in one situation which required rebonding (SR=5.25%). According to the survival estimation method of Kaplan-Meyer the mean survival time was 42.3 months. At the time of evaluation investigated...

Objectives: The aim of this article is to propose a resin cement cleaning protocol for use before recementing a debonded restoration.
Study Design: Ceramic samples were fabricated from IPS d.sign® and IPS e.max Press® and were treated with hydrofluoric acid etching (HF), or HF+silane (S), or HF+S+adhesive or HF+S+A+resin cement. All samples were placed in a furnace at 650º for one minute in order to attempt to pyrolyze the composite. Each step was examined under scanning electron microscopy (SEM).
Results: When the cleaning protocol had been performed, it left a clean and retentive surface.
Conclusions: If the restoration is placed in a furnace at 650º for one minute, the composite cement will burn or pyrolyze and disappear, allowing conventional retreatment of the ceramic before rebonding.

The aim of this study was to evaluate the clinical performance (retention rate) of fiber-reinforced composite fixed partial
dentures (FPDs). Polyethylene fiber(Ribbond??) was used combined with restorative composite during FPDs fabrication. FPDs were placed in thirteen patients in a private clinic. Nineteen FPDS were evaluated. The prosthetic space was filled with only one pontic using extracted teeth (2 cases), acrylic resin teeth (11 cases), or with composite resin (6 cases), combined with Polyethylene fiber. The clinical criterion used was based on retention rate of FPDs. If FPDs were in function in the mouth at the time of examination without previous repair they were classified as Complete Survival (CS) restorations. A classification of Survival with Rebonding (SR) was assigned in the event of an adhesive failure, but after rebonding the FPD still remained under evaluation. Treatment was classified as a Failure (F) if the FPD restoration was lost. The time of evaluation was 41.15 months(??15.13). The FPDs evaluated were retained (CS=94.75%), and no failure was found except for in one situation which required rebonding (SR=5.25%).According to the survival estimation method of Kaplan-Meyer the mean survival time was 42.3 months.
At the time of evaluation investigated...

The microscopic origin and quantum effects of the low barrier hydrogen bond (LBHB) in the proton-bound ammonia dimer cation N2H7+ were studied by means of ab initio and density-functional theory(DFT) methods. These results were analyzed in the framework of vibronic theory and compared to those obtained for the Zundel cation H5O2+. All geometry optimizations carried out using wavefunction-based methods [Hartree–Fock, second and fourth order Möller–Plesset theory (MP2 and MP4), and quadratic configuration interaction with singles and doubles excitations (QCISD)] lead to an asymmetrical H3N–H+⋯NH3 conformation (C3v symmetry) with a small energy barrier (1.26kcal/mol in MP4 and QCISD calculations) between both equivalent minima. The value of this barrier is underestimated in DFT calculations particularly at the local density approximation level where geometry optimization leads to a symmetric H3N⋯H+⋯NH3 structure (D3d point group). The instability of the symmetric D3d structure is shown to originate from the pseudo-Jahn–Teller mixing of the electronic A1g1ground state with five low lying excited states of A2u symmetry through the asymmetric α2u vibrational mode. A molecular orbital study of the pseudo-Jahn–Teller coupling has allowed us to discuss the origin of the proton displacement and the LBHB formation in terms of the polarization of the NH3 molecules and the transfer of electronic charge between the proton and the NH3 units (rebonding). The parallel study of the H5O2+ cation...

The purpose of this study was to evaluate in vitro the shear bond strength of recycled orthodontic brackets. S2C-03Z brackets (Dental Morelli, Brazil) were bonded to the buccal surfaces of 50 extracted human premolars using Concise Orthodontic chemically cured composite resin (3M, USA). The teeth were randomly assigned to 5 groups (n=10), as follows. In group I (control), the bonded brackets remained attached until shear testing (i.e., no debonding/rebonding). In groups II, III and IV, the bonded brackets were detached and rebonded after recycling by 90-mum particle aluminum oxide blasting, silicon carbide stone grinding or an industrial process at a specialized contractor company (Abzil-Lancer, Brazil), respectively. In group V, the bonded brackets were removed and new brackets were bonded to the enamel surface. Shear bond strength was tested in an Instron machine at a crosshead speed of 0.5 mm/min. Data were analyzed statistically by ANOVA and Tukey's test at 5% significance level. There was no statistically significant difference (p>0.05) between the control brackets (0.52 kgf/mm²), brackets recycled by aluminum oxide blasting (0.34 kgf/mm²) and new brackets attached to previously bonded teeth (0.43 kgf/mm²). Brackets recycled by the specialized company (0.28 kgf/mm²) and those recycled by silicon carbide stone grinding (0.14 kgf/mm²) showed the lowest shear strength means and differed statistically from control brackets (0.52 kgf/mm²) (p<0.05). In conclusion...

OBJECTIVE: To compare in vitro shear bond strength (SBS) of different orthodontic adhesives in bonding and repeatedly rebonding metal brackets, and to evaluate the bond failure site with the adhesive remnant index (ARI). METHODS: Specimens consisted of 90 extracted human first premolars, randomly divided into three groups (n=30). The adhesives Alpha Plast (AP), ConciseTM (CO) and TransbondTM XT (TB) were used in each group. Three SBS tests were performed, i.e., one at T0 (initial) and the other two at T1 and T2 (first and second rebondings, respectively), observing a 24-hour interval. The tests were performed in a Shimadzu AG-I (10kN) SBS testing machine, at a speed of 0.5 mm/min. RESULTS: SBS data were subjected to ANOVA, Tukey's test and Bonferroni test (p<0.05). For the ARI, the Kruskal Wallis test was performed, followed by the Dunn test. The results revealed that at T0 groups AP and CO showed SBS values that were near, but above TB values; and at T1 and T2, the highest SBS values were observed for the AP group, followed by the CO and TB groups. CONCLUSION: Statistically significant differences were found in SBS between groups AP, CO and TB during bonding and repeated rebondings of unused metal brackets, with group AP achieving the highest SBS value. Regarding ARI...

Motivated by our previous work using the Stillinger-Weber potential, which
shows that the [$\overline{2}11$] step on 1$\times$1 reconstructed Si(111) has
a Schwoebel barrier of 0.61$\pm$0.07 eV, we calculate here the same barrier
corresponding to two types of kinks on this step - one with rebonding between
upper and lower terrace atoms (type B) and the other without (type A). From the
binding energy of an adatom, without additional relaxation of other atoms, we
find that the Schwoebel barrier must be less than 0.39 eV (0.62 eV) for the
kink of type A (type B). From the true adatom binding energy we determine the
Schwoebel barrier to be 0.15$\pm$0.07eV (0.50$\pm$0.07 eV). The reduction of
the Schwoebel barrier due to the presence of rebonding along the step edge or
kink site is argued to be a robust feature. However, as the true binding energy
plots show discontinuities due to significant movement of atoms at the kink
site, we speculate on the possibility of multi-atom processes having smaller
Schwoebel barriers.; Comment: Manuscript in revtex twocolumn format (7pgs - which includes 14
postscript files). Submitted to the The Journal of Vacuum Science and
Technology (Proceedings of the Physics and Chemistry of Semi- conductor
Interfaces - 23 (1996))

Conventional three-dimensional crystal lattices are terminated by surfaces,
which can demonstrate complex rebonding and rehybridisation, localised strain
and dislocation formation. Two dimensional crystal lattices, of which graphene
is the archetype, are terminated by lines. The additional available dimension
at such interfaces opens up a range of new topological interface possibilities.
We show that graphene sheet edges can adopt a range of topological distortions
depending on their nature. Rehybridisation, local bond reordering, chemical
functionalisation with bulky, charged, or multi-functional groups can lead to
edge buckling to relieve strain, folding, rolling and even tube formation. We
discuss the topological possibilities at a 2D graphene edge, and under what
circumstances we expect different edge topologies to occur. Density functional
calculations are used to explore in more depth different graphene edge types.; Comment: Additional figure in published version

The structural and electronic properties of Si(335)-Au surface decorated with
Pb atoms are studied by means of density-functional theory. The resulting
structural model features Pb atoms bonded to neighboring Si and Au surface
atoms, forming monoatomic chain located 0.2 nm above the surface. The presence
of Pb chain leads to a strong rebonding of Si atoms at the step edge. The fact
that Pb atoms occupy positions in the middle of terrace is consistent with STM
data, and also confirmed by simulated STM images. The calculated band structure
clearly shows one-dimensional metallic character. The calculated electronic
bands remain in very good agreement with photoemission data.; Comment: 8 pages, 8 figures

We present the first ab initio study of the geometry, electronic structure,
charged states, bonding and vibrational modes of the recently synthesized
fullerene-like As@Ni12@As20 cluster which has icosahedral point symmetry
[Science, 300, 778 (2003)]. We show that the molecule is vibrationally stable
and will be electronically most stable in its -3 oxidation state in the
condensed phase and in -2 state in the gas phase. We examine the bonding in
this unusually structured molecule from charge transfer between atoms, infrared
and Raman spectra, and charge density isosurfaces.; Comment: 5 pages, revised version to appear in Physical Review B Rapid
Communication

Self healing mechanisms of vacancy defects in graphene and silicene are
studied using first principles calculations. We investigated host adatom
adsorption, diffusion, vacancy formation and revealed atomistic mechanisms in
the healing of single, double and triple vacancies of single layer graphene and
silicene. Silicon adatom, which is adsorbed to silicene at the top site forms a
dumbbell like structure by pushing one Si atom underneath. The asymmetric
reconstruction of the single vacancy in graphene is induced by the
magnetization through the rebonding of two dangling bonds and acquiring a
significant magnetic moment through remaining unsaturated dangling bond. In
silicene, three two-fold coordinated atoms surrounding the single vacancy
become four-fold coordinated and nonmagnetic through rebonding. The energy
gained through new bond formation becomes the driving force for the
reconstruction. Under the external supply of host atoms, while the vacancy
defects of graphene heal perfectly, Stone-Wales defect can form in the course
of healing of silicene vacancy. The electronic and magnetic properties of
suspended, single layer graphene and silicene are modified by reconstructed
vacancy defects.; Comment: Published in PRB: http://prb.aps.org/abstract/PRB/v88/i4/e045440

The temperature dependent stepwise deprotonation of
5,10,15,20-tetra(p-hydroxyphenyl)porphyrin is investigated using photoelectron
spectroscopy. An abundance of pyrrolic relative to iminic nitrogen and a
decrease in the ratio of the amount of -NH- to -N= with increasing annealing
temperature is found. In contrast to the molecules adsorbed on Au(111), on the
more reactive Ag(110) surface, partial dissociation of the hydroxyl groups and
subsequent diffusion and rebonding of hydrogen to the central nitrogen atoms
resulting in a zwitterionic molecule was clearly observed. Moreover, partial
C-H bond cleavage and the formation of new covalent bonds with adjacent
molecules or the surface starts at a relatively high annealing temperature of
300{\deg}C. This reaction is identified to occur at the carbon atoms of the
pyrrole rings, which leads also to a shift in the N 1s signal and changes in
the valence band of the molecules. Our results show that annealing can
significantly alter the molecules which were deposited depending on the maximum
temperature and the catalytic properties of the specific substrate. The thermal
stability should be considered if a molecular monolayer is prepared from a
multilayer by desorption, or if annealing is applied to enhance the
self-assembly of molecular structures.; Comment: 10 pages...

Fonte: Royal Society of ChemistryPublicador: Royal Society of Chemistry

Tipo: Artigo de Revista Científica

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The widely accepted approach for controlling polymer debonding/rebonding properties in responsive materials has been to purposefully engineer the functional end-groups responsible for monomer dynamic bonding. Here, however, we evidence that the debonding